This invention relates in general to numerical controlled machine tools. More specifically, the present invention provides a numerical controlled machine tool apparatus that is capable of both machining and making measurements of a workpiece during the machining process.
In machining a workpiece by using a machine tool, it is sometimes necessary to measure various dimensions of the workpiece at a time it is partially machined during a machining process in order to get information for modifying the machining process. For example, it may be advantageous to take an intermediate measurement of dimensions such as diameters of a machined bore or coordinate values corresponding to the center of the machined bore to determine how to proceed with further precise or finish machining.
Conventionally, an operator stops the machining process, manually measures the workpiece, and then resumes the remaining machining process making whatever changes in the process are appropriate in view of the measurement information obtained. Heretofore, the sequence of tools used in a machining process has been designated by what is referred to as a "part program" stored in a memory of the numerical control machining apparatus. However that "part program" has not included measuring steps. It has only included machining steps.
With the advent of numerical controlled machine tools (hereinafter referred to as NC machine tool) it has been considered advantageous to automatically carry out an intermediate measuring process by interposing a series of measuring programs for a desired measuring process into a part program used for controlling the machining process. However, in practice, this has been impossible to achieve.
When a workpiece has a plurality of bores to be machined, the measuring process for diameters and center coordinate values corresponding to the machined bores on the workpiece are quite complicated, much more so that the process of merely detecting positions of machined surfaces of a workpiece. To include an automatic measuring process it is necessary to provide in addition to machining tools in a tool magazine at least one measuring probe and machining instructions written on an NC tape (sometimes a "tape" is used as the medium for storing a program to read into an NC apparatus) both for machining and measuring. In addition, a program necessary for executing measured data would have to be added in a processing unit of a numerical control device in order to realize a measuring process as well as a machining process on the same NC machine tool arrangement. Further, as the requirement for machining accuracy becomes higher, the following problem presents itself.
Deviation quantities .DELTA.x, .DELTA.y between a center axis line of a spindle for holding an machining tool and a center axis line of a measuring probe instrument used as a measuring tool, which can be mounted on the spindle, change due to the state with which the measuring tool is mounted on the spindle. For example, there may be an imperfect fit of the measuring probe within the spindle due to the presence of a metal shavings, etc. from previous machining steps. Therefore, if it is desired to determine the coordinate values of the center of a bore in order to machine concentric bores, if the deviation quantities are not predeterminedly measured and compensation is not made, the measured coordinate values of the center of the bore will not be correct due to the center axis line offset.
FIG. 1 illustrates a situation wherein there is an incorrect measurement of coordinates values corresponding to the center of a bore. When a machining process is executed based on such incorrect measurement, a concentric boring process with two steps of machining is unsuccessfully executed. In FIG. 1, a bore 12 with a diameter D1 on a workpiece 11 is machined during a first machining step by using a cutting tool with a diameter D1. Then for getting the bore's center coordinate (lx.sub.o, ly.sub.o), the cutting tool on the spindle is exchanged for a measuring tool mounting a probe 14, and the measuring step is executed.
In case that spindle axis center line L2 is deviated from the true center of bore 12 by .DELTA.x, .DELTA.y as shown in FIG. 1, the coordinate values corresponding to the center of bore 12 are measured as follows: EQU (lx.sub.o -.DELTA.x; ly.sub.o -.DELTA.y)
where lx.sub.o, ly.sub.o designate the true distances of the bored center from the reference surfaces Sx.sub.o, Sy.sub.o of workpiece 11, shown in FIG. 1.
After the measuring step, the spindle axis center line is positioned to L2 by using measured data after the measuring tool on the spindle is exchanged for another cutting tool with a diameter D2, then a second machining step is executed for boring a bore 12A. As shown in FIG. 1, if there are deviation quantities .DELTA.x and .DELTA.y, and the second machining step is executed without taking account of the quantities .DELTA.x and .DELTA.y, in other words, without compensating the measured values lx.sub.o -.DELTA.x and ly.sub.o -.DELTA.y, the result is that bores 12 and 12A are not precisely and concentrically machined.